The present invention relates to a motion detector according to the Ferraris principle with an excitation assembly, an electrically conductive induction element and a sensor assembly.
Motion detectors of this type are known in the art. The excitation assembly of these motion detectors has at least one excitation element capable of applying an inhomogeneous magnetic field to the induction element. When the induction element moves, an eddy current is induced in the induction element due to the motion and the magnetic field. The sensor assembly has at least one sensor element for a measuring a measurement signal which is proportional to the changes in the eddy current induced in the induction element.
For example, DE 37 30 841 A1 or DE 298 13 276 U1 disclose that the excitation element can be excited by applying to the induction element an inhomogeneous time-independent magnetic field. In this case, the changes of the eddy current depend exclusively on the acceleration of the induction element. Accordingly, the measurement signal measured by the sensor element is also proportional to the acceleration of the induction element.
It is also known to apply with the excitation element an inhomogeneous, time-dependent alternating magnetic field to the induction element. Reference is made here to G. Behr: “Alternating current tachometer machines operating according to the Ferraris principle”, ZMSR, Vol. 8, 1959, pp. 355 to 360, and E. Habiger: “Foundation for dimensioning the Ferraris machines as a control element”, Elektrie, Vol. 6 (1964), pp. 170 to 178. The temporal changes of the eddy current are here dependent both on the velocity and on the acceleration of the induction element. With a suitable selection of the dimensions, in particular with a suitable choice of the excitation frequency, the measurement signal can be dominated by the velocity contribution. An acceleration component, however, is always present.
It would therefore be desirable and advantageous to provide a motion detector according to the Ferraris principle, which obviates prior art shortcomings and produces a useful signal which is truly proportional to either the velocity or to the acceleration of the induction element.